XMPP

On this topic deltachat has come up with a beautiful solution I believe. Your identity is your public key, and servers are used solely to relay stuff from one end to the other. You can have 10 "accounts" on 10 different servers, servers can fall and it's still ok because there are 9 still up.

The con is that you don't have a simple, publicly usable pointer to you like a mail address or a phone number. Turns out it's the best defense against spam, creepy weirdos, trolls, so it's not that problematic.